The present invention relates to a circuit for executing an interpolation processing on a sub-sampled image signal, and more particularly to improvements in a circuit in which, after a first image signal primitively sampled in the form of a square lattice has been limited in band area by a prefilter and then sub-sampled as inter-frame offset in the form of a quincunx to generate a second image signal, an interpolation processing is executed on the second image signal in moving areas.
As a conventional technique of compressing the band area of an image signal, there is known the sub-sampling technique. According to this sub-sampling technique, a primitively sampled digital image signal is further repeatedly subjected to a sampling processing to reduce the image signal in the amount of information, thereby to compress the band area thereof. The image signal thus compressed in band area by this sub-sampling technique is to be transmitted in a narrow transmission band area from the transmitting side such as a TV station or the like to a TV receiving side through a transmission system.
FIG. 13 shows the arrangement of the transmitting side such as a TV station.
Shown in FIG. 13 are an A/D converting circuit 70 for converting an image signal from a television camera or the like from an analog signal to a digital signal, a stationary area prefilter 71 and a moving area prefilter 72 for executing a prefilter processing on the digital image signal from the A/D converting circuit 70, and an inter-field offset sampling circuit 73 for subjecting the image signal from the stationary area prefilter 71 to an inter-field offset sampling.
A moving area detector circuit 74 is disposed for detecting, based on the image signal from the moving area prefilter 72, whether or not the picture area formed by the image signal is a moving area. A mixing device 75 is arranged such that, based on the result of detection by the moving area detector circuit 74, the mixing device 75 either selects an output of the moving area prefilter 72 or an output of the inter-field offset sub-sampling circuit 73, or mixes the both outputs of the circuits 72, 73 at a predetermined mixing ratio. An inter-frame offset sub-sampling circuit 76 is disposed for subjecting an image signal supplied form the mixing device 75 to inter-frame offset sub-sampling, thereby to compress the band area of the image signal.
In a stationary area, the image signal is subjected to a prefilter processing using the stationary area prefilter 71 and then subjected to inter-field offset sub-sampling and inter-frame offset sub-sampling such that the information of the image signal is compressed. In a moving area, the image signal is subjected to a prefilter processing using the moving area prefilter 72 and then subjected to inter-frame offset sub-sampling such that the information of the image signal is compressed.
Further, a D/A converting circuit 77 is disposed for converting the image signal from the inter-frame offset sub-sampling circuit 76 from a digital signal to an analog signal. An output circuit 78 is disposed for externally supplying an image signal from the D/A converting circuit 77.
When the image signal thus sub-sampled at the transmitting side is to be received by a television receiver or the like, the television receiver or the like has, as a circuit for reproducing a picture from the received signal, a circuit for executing an interpolation processing on the image signal.
As shown in "MUSE-High Vision Transmission System" (Yuichi NINOMIYA, Institute of Electronics, Information and Communication Engineers, 1990), such an image signal interpolation processing circuit has two different picture reproducing methods for the case where the original picture is stationary and the case where the original picture is moving. In the stationary areas, the original picture is reproduced by executing an inter-frame interpolation using a plurality of frames. In the moving areas, a 1-frame signal is subjected to a processing of sampling rate conversion, and then subjected to an interpolation processing using a low-pass filter, thereby to reproduce a picture from the only 1-frame image signal.
FIG. 14 shows the arrangement above-mentioned of a television receiver or the like.
Shown in FIG. 14 are an A/D converting circuit 80 for converting a received image signal from an analog signal to a digital signal, a frame interpolation circuit 81 for subjecting a digital image signal from the A/D converting circuit 80 to a frame interpolation, a field interpolation circuit 82 for subjecting an image signal interpolated by the frame interpolation circuit 81 to a field interpolation, and an intra-field interpolation circuit 83 for subjecting a digital image signal from the A/D converting circuit 80 to intra-field interpolation.
A moving area detector circuit 84 is disposed for detecting, based on the image signal form the A/D converting circuit 80, whether or not the picture area formed by the image signal is a moving area. A mixing device 85 is arranged such that, based on the result of detection by the moving area detector circuit 84, the mixing device 85 either selects an image signal from the field interpolation circuit 82 or an image signal from the intra-field interpolation circuit 83, or mixes the both image signals of the circuits 82, 83 at a predetermined mixing ratio. An output circuit 86 is disposed for externally supplying an image signal supplied from the mixing device 85 to a TV receiver or the like.
In the moving area, there is used, in the sub-sampling circuit, a prefilter through which the image signal passes only at the transmissible area of the picture, thereby to limit the image signal in band area, thus preventing aliasing interference of the signal. Accordingly, a stationary area is transmitted without the amount of information reduced, but a moving area is reduced in the amount of information due to limitation of band area. However, a man's visual sensation is lower to a moving area than to a stationary area. Therefore, a reduction in resolution of a motion picture is hardly sensed and presents no trouble.
With reference to attached drawings, the following description will discuss an example of a conventional circuit for executing an interpolation processing on all image signal when processing such a motion picture as above-mentioned.
First, there is discussed how to generate and transmit an image signal to be received by the interpolation processing circuit. At the image-signal transmitting side such as a TV station or the like, a primitive sampling signal is entered at a signal input portion and then limited in band area by a prefilter comprising a low-pass filter portion to eliminate the high frequency component contained in the primitive sampling signal. Thereafter, the signal is subjected to a sampling processing at a sub-sampling portion.
The primitive sampling signal is an image signal sub-sampled in the form of a square lattice in which the horizontal direction is expressed in terms of h(Hz) and the vertical direction is expressed in terms of v(TV piece). As shown in FIG. 6 according to the sampling theorem, the band area of transmissible frequency of the primitive sampling signal is defined by the horizontal spatial frequency of h/2(Hz) and the vertical spatial frequency of v/2 (TV piece).
The following description will discuss a case where the primitive sampling signal is compressed in band area to 1/2 by sub-sampling. At a sub-sampling portion, out of sampling points 51 of the primitive sampling signal in FIG. 5, sampling points 52 are selected in the form of a quincunx for the even-numbered frames and sampling points 53 are selected, as shown in FIGS. 7(a) and (b), respectively. Thus, the primitive sampling signal is transmitted as compressed in band area to 1/2.
FIG. 15 shows a block diagram of a conventional interpolation processing circuit.
Shown in FIG. 15 are a transmission system 90 to which an image signal compressed in band area is to be transmitted from a TV station or the like. A sampling rate conversion portion 91 is disposed for inserting a zero value to each of the interpolation points of an image signal received from the transmission system 90. A low-pass filter portion 92 is disposed for executing an interpolation processing on an image signal from the sampling rate conversion portion 91. A signal output portion 93 is disposed for supplying an image signal from the low-pass filter portion 92. A Braun tube 94 is disposed for displaying a picture upon reception of an image signal from the signal output portion 93.
In the interpolation processing circuit having the arrangement above-mentioned, after an image signal compressed in band area has been received from the transmission system 90, the sampling rate conversion portion 91 is operated to put, (i) in the even-numbered frames of the image signal, zero into each of other sampling points than the sampling points 52 in the even-numbered frames in the form of a quincunx shown in FIG. 7(a), i.e., the points corresponding to the sampling points 53 in the odd-numbered frames in FIG. 7(b), and (ii) in the odd-numbered frames of the image signal, zero into each of other sampling points than the sampling points 53 in the odd-numbered frames in FIG. 7(b), i.e., the points corresponding to the sampling points 52 in the even-numbered frames in FIG. 7(a). Thus, the sampling frequency of the image signal is returned to the original one. Thereafter, the signal thus subjected to sampling rate conversion is subjected to a low-pass filter using the low-pass filter portion 92 such that an interpolation processing is executed. The image signal thus interpolation-processed is then supplied from the signal output portion 93 and reproduced on the externally disposed Braun tube 94.
As shown in "Consideration about a Chrominance Signal Processing for MUSE System" (the Transactions of the Institute of Electronics, Information and Communication Engineers B-1, vol. j76-B-1, No. 3, pp. 290-298, March, 1993), a prefilter used in sub-sampling is changed in filter configuration to improve a reproduced image in quality. The low-pass filter used in interpolation is also changed in filter configuration according to the change in filter configuration of the prefilter used in sub-sampling.
The transmissible signal band area in the transmission system depends on the sub-sampling circuit at the TV station side and on the low-pass filter in the interpolation processing circuit at the image-signal receiving side. For example, when a vertical filter is used in the sub-sampling circuit 76 in FIG. 13, the vertical spatial frequency is limited to v/4 (TV piece) or a half as compared with that of the primitive sampling signal. When a two dimensional low-pass filter is used, the transmissible signal band area is triangular as shown in FIG. 9. It is desired that the spatial frequency characteristics of the low-pass filter portion 92 in the interpolation processing circuit are identical with or approximate to, with high precision, the spatial frequency characteristics of a low-pass filter portion 2 in the sub-sampling circuit at the TV station side.
However, there are instances where, due to improvements and advances in the technique of a TV receiver or the like, the transmissible band area at the TV receiver side is not identical with or approximate to, with high precision, the transmissible band area at the TV station side. As a result, there may be a case for example where there is used, as the low-pass filter portion 92 in the interpolation processing circuit at the image-signal receiving side, a vertical low-pass filter presenting a rectangular transmissible band area shown in FIG. 8 although there is used, as the sub-sampling circuit 76 at the image-signal transmitting side, a two dimensional low-pass filter presenting a triangular transmissible band area shown in FIG. 9. In such a case, a band area A1 shown in FIG. 10 is folded down to a band area A2 at the image-signal receiving side. This disadvantageously produces, for example, interference that a horizontal straight line of a reproduced picture is converted into a broken line.